Wood-polymer composites comprising a thermoplastic resin and wood flour have been conventionally known. Wood-polymer composites are thermoplastic as compared to products made from lumber alone, and advantageously processed into various shapes. Accordingly, they are widely used for house construction members, building materials and the like, besides lacquer wares, doors, frames and the like. A wood-polymer composite, when seen as a thermoplastic resin, is advantageous in that it affords woody texture, such as appearance (e.g., color tone, gloss etc.), a touch (e.g., thermal conduction, surface roughness) and the like, which are characteristic of lumber, decreases coefficient of thermal expansion, is light weight as compared to inorganic filler products, and so on. Moreover, end lumber pieces, scrap wood, sawdust and the like, which are produced in the lumbering industry, can be used as the wood flour for wood-polymer composites, thereby enabling effective utilization of non-used resources.
As regards the difference between thermoplastic resin and wood-polymer composite in mechanical strength, there is a report wherein a wood-polymer composite comprising polyvinyl chloride (PVC) and 30% of wood flour, and a molded product made from PVC alone were compared. The report concludes that the wood-polymer composite is superior to molded products made from PVC alone in flexural modulus but inferior thereto in tensile strength, bending strength, elongation at break and impact strength (Iwasaki: Wood Industry, 40(5), p 203-208 (1985)). In general, an interfacial binding force between thermoplastic resin and wood flour is weak, and the above-mentioned degradation of tensile strength, bending strength, elongation at break and impact strength is considered to be attributable to such weak interfacial binding force. To improve this, various methods have been heretofore proposed, such as the use of a resin wherein a functional group capable of reaction with hydroxyl group on the surface of wood flour is introduced, such as maleic anhydride modified polypropylene (Shiraishi: Polymer Application, 38(7), p 338-344, (1989)), surface treatment of wood flour with a silane coupling agent (Ide: Surface Control and Design of Composite, Sigma Shuppan, p 148-169) and the like. Nevertheless, none of the proposed methods has succeeded in producing a wood-polymer composite showing well-balanced mechanical strength and processability as compared to products made from lumber alone and products made from thermoplastic resin alone.